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The submarine cable network
Cable routing

The submarine cable network

Since the end of the nineteen-eighties, the demand for telecommunications resources has been accelerating. This growth is set to continue in the years to come.
Telecommunication via submarine cable is of vital significance for the world economy. submarine cables carry the bulk of intercontinental telecommunications, but also communication between countries that are relatively close to one another for various reasons (relief, towns etc), as well as between mainland communities and islands offshore.

Intercontinental Traffic

  • 70% of all traffic is routed via submarine cable
About twenty international submarine links land in metropolis in seven terminal stations:
  • 3 on the Mediterranean coast, supplying the Mediterranean basin : the Middle East, East Africa and South East Asia.
  • 2 on the Atlantic coast linking to North America, Portugal, Morocco, Western Africa, South Africa and South America.
  • 2 in the Channel, linking to the United Kingdom.
Half of these routes have France as their destination, the other half extending out to other European countries or to the Mediterranean basin, making France a platform or focal point for intercontinental traffic.

French provincial states such as Guadeloupe, Martinique, St Martin and Guyana are linked to other islands in the Caribbean, and to the American continent. The island of Reunion will soon be linked to Africa, Asia and Europe.

Submarine cable telecommunications systems
The ability to convey voice across the oceans became a reality in the 1950's. It quickly overtook the telegraphic transmission which had been operational for nearly a century, and gradually replaced it altogether.

Satellite transmission has existed since the 1960's. Until the 1990's, the submarine cable was the preferred means of communicating for short, high-volume links and satellite transmission was used mainly for long distance, low volume links.
Nevertheless, for long distance high-volume links, cable and satellite are complementary.

From 1988 onwards, fibre optic transmission is exploited in submarine cables. The first transmissions via fibre optic cable had flow rates of 280 Mbit/second, then 560 Mbit/s, and today are of the order of Terabit/sec.

At the dawn of the third millennium, all analogue submarine links will be replaced by fibre optic cabling supporting digital transmission.

Main elements of a submarine fibre optic cable
We can break these down into two main groups :
  1. submerged equipment
  2. Terminal station installations.
a. submerged equipment
The fibres transporting the flow, run through the centre of an optical module. The optical module is itself encapsulated by a layer of steel, a covering of copper, and finally a polyethylene casing. The steel sheath or covering gives the cable its mechanical robustness, while the copper envelope makes it possible to convey the current from the power feed equipment. The polyethylene sheath isolates the inner conductor cable from the marine medium. The structure as defined above constitutes the cable known as the deep sea cable.

By adding steel wires, thread and bitumen, the cable is reinforced to increase its resistivity to any possible damage in shallow waters (anchors, trawling gear, dragnets, currents).

Repeaters re emit the signal transmitted so as to compensate for losses due to the long-distance propagation along the fibres. They occur at regular intervals along trans-oceanic cables (every 5,000 - 10,000 kms) and medium-distance links.

Branching units
These units make it possible to take branches off the main cable so as to supply multiple cable terminal stations, and so a wider area on land.

b. Terminal station installations
power-feeding equipment
These provides energy for the signal boosters contained in the repeaters.

Line terminals
These amplify and format signals, and control the transmission quality.

Fault location equipment
Location of faulty or defective cable (cut, stretched, or damaged protective covering.)


Cable routing

Formerly, offshore positioning for cable ships laying their cables was calculated using the sextant, and so with considerable potential for error.

The old telegraphic cables laid more than one hundred years ago were put down with very little guarantee of any accuracy in their positioning.

As soon as it became possible, radio-electric positioning was used. Since the GPS (Global Positioning System) became generally accessible, all surveys carried out prior to laying cables, and then the final cable laying itself, have been undertaken with excellent levels of precision, and this was the case for any area in the world.

All positions are now determined using Differential GPS, which is today the best positioning system available, and can achieve precision of the order of ten metres.
It should be noted that only cables laid since the 1980's benefit from this technique. The routes of older communication channels are not known with any degree of accuracy.

Many cable sections have since been lifted. This is why Orange does not include them in their data bases, nor are they indicated on charts or maps.

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